The present invention is directed to surgical apparatus and associated methods for measuring the size of a valve annulus (that is, the opening resulting from the removal of a diseased natural valve) during heart valve-replacement surgery. Valve annuluses need to be sized in order for a surgeon to select a properly sized replacement artificial valve.
The heart has four valvesxe2x80x94two on the right (the pulmonic and tricuspid) and two on the left (the aortic and mitraly)xe2x80x94that control the flow of blood through the chambers of the heart and out to the body. Although any of these valves may fail to function properly, disease most commonly affects the valves on the left side of the heart. The valves may narrow (called stenosis); the valves may not close all the way (causing a backflow of blood called regurgitation); or the valves may close incorrectly (called prolapse). A heart murmur represents the sound that a leaky or narrowed heart valve makes as blood moves through it.
The Aortic and Mitral Valves
Aortic stenosis is a narrowing of the aortic valve, through which blood flows from the left ventricle of the heart to the ascending aorta, the major artery whose branches supply blood to various parts of the body. Sometimes this narrowness is a congenital (i.e., inborn) defect, but more often the valve narrows as a consequence of aging, or of infections, such as rheumatic fever. Aortic stenosis results in the left ventricle having to work harder and harder to push blood out. As this occurs, the muscular walls of the ventricle thicken, increasing their requirement for oxygen. Symptoms of aortic stenosis include chest pain when the oxygen needs exceed the supply from the coronary arteries; fainting (syncope), if the valve becomes very tight; and congestive heart failure, which usually does not occur unless the valve has been narrowed for many years. Valve replacement, either with a mechanical or tissue valve often alleviates these symptoms.
In mitral stenosis, the valve opening between the upper and lower chambers on the left side of the heart has become narrowed. The cause is almost always rheumatic fever, which is now rare in most developed countries but is common in many parts of the world. When mitral stenosis occurs, the entry of blood into the left ventricle from the atrium is impeded by the narrow valve. Pressure builds up behind the valve, leading to an elevation of pressure in the lungs. This in turn may lead to shortness or breath (dyspnea), which is one of the major symptoms of mitral stenosis. Often, however, it occurs without any symptoms.
In aortic regurgitation, the aortic valve fails to close completely after the heart has pumped blood out into the aorta. Blood leaks back from the aorta into the left ventricle. In mitral regurgitation, improper closure causes blood to lead from the left ventricle back into the left atrium. In either case, the valve does not close properly because of a physical change in its shape or its support. This change may be the result of rheumatic fever; an infection (endocarditis), which may leave the valve scarred; or a heart attack, which causes loss of supporting muscle tissue. In the mitral valve, the change may be the result of a heart attack, which causes a loss of muscle tissue, or a spontaneous rupture of one of its muscular chords (chordea tendineae) that normally act as guide wires to keep it in place.
Major symptoms of defective mitral valves include fatigue, shortness of breath, and edema. Medications such as digitalis, diuretics, and angiotensin-converting enzyme (ACE) inhibitors can help alleviate symptoms. Some defective mitral valves can be reconstructed or, failing that, replaced by an artificial valve.
The Pulmonic and Tricuspid Valves
In the pulnonic and tricuspid valves, any narrowing is rare and almost always congenital. Leakage, or regurgitation, is unusual, but may occur when use of illicit intravenous drugs leads to infection that damages the valve. The infection, hallmarked by fever, often settles on these two valves because they are the first ones bacteria come in contact with as they travel through the bloodstream. If the valve becomes leaky, swelling of the abdomen and legs may occur. As with other valves, treatment can include replacement, but this is rare and usually not as effective as it is when the aortic or mitral valve is involved.
Artificial Valves
Valve-replacement surgery is usually recommended when the damage to the valve is severe enough to be potentially life-threatening, as in the case of severe aortic stenosis. The mitral and aortic valves are the heart valves that most often need to be replaced. Artificial valves have been in use since 1952, when Charles Hufnagel successfully replaced a patient""s aortic valve with a caged-ball valve.
There are two types of artificial, or prosthetic, valves that can be used to replace the original valves: mechanical and tissue. Mechanical valves are made of synthetic materials, such as metal alloys, carbon, and various plastics. They come in two major designs: a xe2x80x9ccaged-ball valvexe2x80x9d and a xe2x80x9ctilting-disk valve.xe2x80x9d Tissue valves can be composed of animal or human valve tissue. Because of the scarcity of human valves available for transplantation, pig valves, specially processed and sutured into a synthetic cloth, are most often used. These valves are also called porcine valves. Pericardial valves make use of leaflets cut from the pericardium sac of a cow. Most tissue valves are well tolerated by the human body and are much less likely to require blood-thinning therapy, but they tend to be less durable: after 10 years, some 60 percent need to be replaced.
Both mechanical and tissue valves include some support structure or stent and a soft peripheral sewing ring. The sewing ring is used to secure the valve into place occluding the annulus, and must provide a good seal around the valve to prevent leakage.
Valve Replacement Surgery
Valve replacement is performed during open-heart surgery. The valves are mounted in an annulus comprising dense fibrous rings attached either directly or indirectly to the atrial and ventricular muscle fibers. In a valve replacement operation, the damaged leaflets are excised and the annulus sculpted to receive a replacement valve. Ideally the annulus presents relatively healthy tissue which can be formed by the surgeon into a uniform ledge projecting into the orifice left by the removed valve. The time and spatial constraints imposed by surgery, however, often dictate that the shape of the resulting annulus is less than perfect for attachment of a sewing ring. Moreover, the annulus may be calcified as well as the leaflets and complete annular debridement, or removal of the hardened tissue, results in a larger orifice and less defined annulus ledge to which to attach the sewing ring. In short, the contours of the resulting annulus vary widely after the natural valve has been excised.
The annulus is sized with an annulus sizer to determine the proper size of the replacement artificial valve. The artificial valve is then positioned in the opening and the sewing ring is carefully sutured or sewn to the tissue surrounding the valve opening. Given the uneven nature of the annuluses, the match between the valve sewing ring and annulus is a crucial aspect of prosthetic heart valve implantation. The annulus sizer is typically cylindrical, and made of hard plastic with a central threaded tap to which a handle is attached. A number of sizers are at a surgeon""s disposal, each having a different size, or diameter. In use the surgeon inserts the sizer into the valve opening, measuring the size of the opening. An artificial valve properly sized for the valve opening is then selected and sewn in place.
Most annulus sizers are made from a biocompatible material and are rigid and inflexible. In contrast, the sewing rings of artificial valves are flexible. When inserted in the valve opening, the sewing ring may compress. The compression may result in the valve being too small for the valve opening. If this happens, the valve needs to be discarded, and a new valve needs to be chosen. As artificial valves are expensive to produce, discarding an artificial valve unnecessarily represents a tremendous waste. Also the time which is wasted in replacing improperly sized valves during the valve replacement surgery is critical to the patient and should avoided. Another possible error in sizing stems from using the rigid circular sizer to measure what is often an irregular annulus.
Accordingly, in view of the foregoing, it is an object of the present invention to provide annulus sizers which eliminate many of the drawbacks associated with conventional sizers.
It is thus an object of the present invention to provide annulus sizers which enable a surgeon to accurately select a properly sized artificial valve.
It is yet another object of the present invention to provide annulus sizers for measuring the size of valve openings which mimic the physical characteristics of an artificial valve sewing ring.
It is still another object of the present invention to provide methodology which enables surgeons to accurately determine the size of valve annuluses which, in turn, enables surgeons to select properly sized replacement artificial valves during valve replacement surgery.
These and other objects are achieved by the surgical apparatus and associated methods of the present invention which enable a surgeon to accurately measure the size of a valve annulus and then to properly selected a replacement artificial valve during valve-replacement surgery.
In accordance with a broad aspect of the invention, a sizer for measuring a valve annulus to determine a size of an artificial heart valve to be sewn in the valve annulus during heart-valve replacement surgery, includes a support member and a resilient member. The support member has a size corresponding to the size of one of a plurality of artificial heart valves. The resilient member is disposed about the support member and has a resiliency substantially equal to the resiliency of a sewing ring of the artificial heart valve. Accordingly, when a surgeon inserts the sizer into a valve annulus, the resilient member conforms to the shape of the valve annulus, analogous to how the sewing ring will conform when positioned in the annulus and sewn in place. The surgeon is therefore able to determine more accurately the size of the annulus and, thereafter, to select a properly sized artificial valve.
In addition to having substantially the same resilience as the sewing ring of an artificial heart valve, the resilient member also preferably is configured substantially the same as the sewing ring of the artificial heart valve. By substantially matching the artificial heart valve configuration, the sizer is able to xe2x80x9cmimicxe2x80x9d more accurately how the artificial heart valve will be received in the valve annulus for sewing.
The support member of the sizer is preferably releasably attachable to a surgical handle. Accordingly, in the operating theater, a surgeon is able to select a sizer and insert the sizer into the valve annulus to determine the size of the annulus. If the sizer does not fit to the surgeon""s satisfaction, the surgeon is able to remove the sizer from the annulus, detach the sizer from the handle, select and attach another sizer of different size, and re-insert the new sizer into the annulus. This process may be repeated until the surgeon has determined the size of the valve annulus.
Other aspects, features, and advantages of the present invention will become apparent to those persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings.